Probably the best way to diagnose would be to boot an Ubuntu CD as a live CD (select Try Ubuntu without installing) and run the bootinfoscript.

You can also just hit CtrlAltF1 when you get that error to drop to a terminal and run:
sudo parted -l and sudo fdisk -l
The first will tell you if there are partition table errors. The second will tell you if there is some leftover GPT data (usually if you reuse a disk from a Mac). If you need to remove this GPT data you can use fixparts found here.

For example, this sudo fdisk -l output shows that a GUID partition table is detected, but does not seem to be used: